[Telemetry] Rename interaction_record field of value to tir_label.

[Telemetry] Rename interaction_record field of value to tir_label.

The name 'interaction_record' does not make it sufficiently clear that the
so-named field of Value is to be used to indicate the label of the
TimelineInteractionRecord to which it pertains. This CL renames it 'tir_label'
to make this distinction clear.

R=nednguyen
BUG=461101
Committed: https://crrev.com/0f8bf3b33319a20fbe475f72c4a0d91d742c5252
Cr-Commit-Position: refs/heads/master@{#323848}

[eakuefner, 2015-04-03 22:41:12]

PTAL, simple renaming-only change

[nednguyen, 2015-04-03 22:48:39]

lgtm

[eakuefner, 2015-04-03 22:48:54]

The CQ bit was checked by eakuefner@chromium.org

[commit-bot: I haz the power, 2015-04-03 22:49:11]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1057213005/1

[commit-bot: I haz the power, 2015-04-03 23:43:23]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-04-03 23:44:10]

Patchset 1 (id:??) landed as
https://crrev.com/0f8bf3b33319a20fbe475f72c4a0d91d742c5252
Cr-Commit-Position: refs/heads/master@{#323848}